1. Field of the Invention
The present invention relates to a leaky coaxial cable in which the outer diameter of an insulator is below 10 mm.
2. Description of the Related Art
As described in Kishimoto, T. and Sasaki, S., 1982, LCX Tsuushin Sisutemu (LCX Communications System), 1st ed., Corona Publishing Co., Ltd., Tokyo, Japan, a leaky coaxial cable (LCX) is a cable which is designed to radiate part of electric signal energy to be transmitted inside the cable to the outside in the form of electromagnetic waves. The LCX is used as a transmission and reception antenna in a radio communication system. The LCX is installed along railroad tracks for radio communications between trains and the ground, for example. The LCX is also installed in subway stations or underground malls for fire radio communications or police radio communications to and from the subway stations or the underground malls.
A conventional LCX is shown in FIG. 1. As shown in the drawing, the LCX is formed as a coaxial cable which includes: a central conductor 201; an insulator 202 covering the central conductor 201; an external conductor 203 located around this insulator 202; and an outer sheath 205 covering this external conductor 203. The material of the central conductor 201 and the external conductor 203 is usually copper, and aluminum is sometimes used as well. The material of the insulator 202 is Polyethylene, for example.
The external conductor 203 of the LCX has slots 206 serving as electromagnetic wave leak mechanisms. The slots 206 are provided periodically in the longitudinal direction of the cable. Each slot is an opening having an elongated shape or a round shape.
A type name of the LCX is generally expressed by using an outer diameter of the insulator and the characteristic impedance (standard impedance) of the LCX. For example, if the LCX includes the insulator with an outer diameter of 20 mm and has impedance of 50Ω, then the LCX is expressed as 20D type. The LCX conventionally includes 20D type, 33D type, 43D type and so forth, and the outer diameters of the outer sheaths thereof are as extremely large as 30 mm, 40 mm, and 50 mm, respectively. In the meantime, the external conductor needs to be thick enough not to stretch or crack even when traction force or bending force is applied thereto at the time of outdoor installation. To be more specific, this thickness is approximately from 0.1 mm to 0.2 mm in consideration of material costs as well.
Japanese Patent Application Laid-Open Publication Nos. 10-193001 and 2003-179415 describe methods of forming the slots 206 in the external conductor 203. The former discloses press work using male and female dies formed in conformity to the shape of the slots 206, and the latter discloses formation by means of laser beam irradiation. As another formation method, cutting work using an end mill is proposed.